In ordinary video encoding methods, motion prediction is performed by means of block matching based on minimization of the differences between pixel values between frames, so as to improve encoding performance. However, in an image (having a fading effect or the like) whose brightness temporally varies, the prediction residual increases in accordance with the variation in the brightness, which degrades the encoding performance.
Therefore, H.264/AVC (see Non-Patent Document 1) employs weighted motion prediction for adaptively multiplying a reference picture for the motion prediction by a weighting coefficient. A predicted reference signal, in which the temporal variation in brightness has been corrected, is obtained due to the above weighted motion prediction, thereby improving the encoding performance.
The weighted motion prediction in H.264/AVC has two coefficient transmission modes, such as (i) an Explicit mode for encoding and transmitting the weighting coefficient, and (ii) Implicit mode for indirectly generating a common weighting coefficient between the encoder and the decoder based on information about the reference frame. Table 1 shows the types and prediction methods of the weighted motion prediction for P and B slices.
TABLE 1Type and methods of weighted motion prediction in H.264/AVCTypePrediction typepredicted signalcoefficient transmissionP slice—z = w0 · y0 + d0transmit w0 and d0 (Explicit)B sliceL0/L1 predictionz = w0 · y0 + d0 (L0 prediction)transmit w0, d0, w1, and d1z = w1 · y1 + d1 (L1 prediction)(Explicit)bi-predictivez = w0 · y0 + w1 · y1 + dtransmit w0, d0, w1, and d1prediction(d = ½(d0 + d1))(Explicit)z = w0 · y0 + w1 · y1 + dcoefficient computation based ondistance from reference picture(Implicit)
In Table 1, z represents a weighted motion-predicted signal, y, y0, and y1 represent reference signals for the weighted motion prediction, and w, w0, w1, d, d0 and d1 represents weighting coefficients. Switching operation for the weighted motion prediction and mode selection for the coefficient transmission are performed for each slice.
FIG. 1 is a diagram used for explaining a weighted motion prediction (Implicit mode) in H.264/AVC. The Implicit mode is applied only to the bi-predictive prediction for B slices, and the weighting coefficients are generated in the relevant encoder and decoder, as shown in FIG. 1, where tb and td indicate distances between frames.
Here, it is assumed that the variation in brightness between an encoding target frame and two reference frames is linear. The proportional coefficients w0 and w1 are computed in accordance with the distance from the reference picture. The offset coefficient d is defined to be 0.w0=1−w1 w1=tb/td d=0
Although FIG. 1 shows an example of proportional coefficient computation by means of interior division in accordance with the distance from the reference picture, a similar operation can be performed when exterior division is employed. If the brightness of the relevant image varies linearly, appropriate weighting coefficients can be computed even in the Implicit mode, and the amount of code required for coefficient information can be reduced.
The same weighted motion prediction is currently employed by JSVC (an extended scalable method) of H.264/AVEX (see Non-Patent Document 2), which is examined in JTD (a joint party of ISO and ITU-T).
The weighted motion prediction shown in Table 1 is also employed in the reference encoder JSVM (program) shown in Non-Patent Document 3.
A technique shown in Patent Document 1 is also known, in which a general variation in luminance of a video image is detected so as to perform luminance compensation. In order to handle a case in which the brightness variation is not constant over the entire image, the technique of Patent Document 1 uses an amount of general variation in luminance over the entire image and applies a flag which indicates whether or not the luminance variation is compensated in each small area.
Non-Patent Document 1: ITU-T: “Advanced video coding for generic audiovisual services”, ITU-T Rec, 11.264, pp. 129-132, 2003.
Non-Patent Document 2: T. Wiegand, G. Sullivan, J. Reichel, H. Schwarz and M. Wien: “Joint Draft 9 of SVC Amendment”, ISO/IFC JTC1/SC29/WG11 and ITU-T SG16 Q.6, JVT-V201, pp. 166-170, 2006.
Non-Patent Document 3: J. Reichel, H. Schwarz and M. Wien: “Joint Scalable Video Model JSVM-8.0”, ISO/IEC JTC1/SC29/WG11 and ITU-T SG16 Q.6, JVT-U202, 2006.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H10-32824.